This invention relates generally to signaling systems in which a plurality of remote stations are connected to a central station by common signal transmission paths. More specifically, the invention relates to signaling systems of this type in which information from the remote stations is transmitted to the central station by time-division multiplexing use of a common signal transmission path. This is to be distinguished, of course, from a signaling system in which each of a plurality of remote stations is separately wired to a central station, such as in conventional security alarm systems.
The broad principle of employing cascaded switching devices to detect conditions at a plurality of remote locations is not new. In accordance with the principle, a stimulus currrent or voltage is applied by the central station to a string of switching devices, which respond sequentially in a predictable manner, unless an abnormal condition causes a different response to be generated and detected. British Pat. Nos. 1,027,726 and 1,001,083 describe apparatus that falls into the same category as this broad principle. The same principle is disclosed in Swiss Pat. No. 473 432, and in German Pat. Nos. 2638068, 2533382, 2533354, and 2635763. Gernam Specification 2638068 corresponds to U.S. Pat. No. 4,162,498.
U.S. Pat. Nos. 4,206,449 and 4,359,721 to Galvin et al. are examples of two-wire alarm systems, and the one described in U.S. Pat. No. 4,359,721 employes a cascading principle. Nevertheless, the Galvin system is not of the same type as the present invention, since each remote station in the Galvin system has to identify itself uniquely by the nature of its response. As will be explained in detail, the remote stations in the system of the invention are identical units and are identifiable only by their position in a string of remote stations.
Prior to this invention, the cascade principle as disclosed in the patent publications listed above had a number of shortcomings. First, as the number of remote stations is increased the cumulative current drawn by the stations in series becomes significantly large, and there is a significant voltage drop between the central station and the most remote station. Another disadvantage of cascade systems of the prior art is that it is possible to sense only one variable at each remote station, either by modulating the time or magnitude of a current pulse. These and other disadvantages of the prior art are overcome by the signaling system of the present invention.